Polymorphism using UxmlAttribute

UxmlAttribute_SerializeReference.cs

using System;
using UnityEditor;
using UnityEditor.UIElements;
using UnityEngine;
using UnityEngine.UIElements;

// Our Interface.
public interface IMyInterface
{
}

// Example implementations of the interface.
[Serializable]
public class MyInterfaceImplementation : IMyInterface
{
    public int someValue;
}

[Serializable]
public class MyOtherInterfaceImplementation : IMyInterface
{
    public string someString;
    public float someFloat;
}

// Our class that contains a reference to the interface.
[Serializable]
public class MyClass
{
    [SerializeReference]
    public IMyInterface myInterface;
}

// The converter class. This converts to and from the string representation of the interface instance
public class MyClassConverter : UxmlAttributeConverter<MyClass>
{
    public override MyClass FromString(string value)
    {
        return JsonUtility.FromJson<MyClass>(value);
    }

    public override string ToString(MyClass value)
    {
        return JsonUtility.ToJson(value);
    }
}

// The UXML element that uses the MyClass type.
[UxmlElement]
public partial class MyElement : VisualElement
{
    [UxmlAttribute]
    public MyClass myClass { get; set; }
}

// The property drawer for the MyClass type. So we can give users options on what type to create.
[CustomPropertyDrawer(typeof(MyClass))]
public class MyDrawer : PropertyDrawer
{
    public override VisualElement CreatePropertyGUI(SerializedProperty property)
    {
        var p = property.FindPropertyRelative("myInterface");

        var root = new VisualElement();

        root.Add(new Button(() =>
        {
            p.managedReferenceValue = new MyInterfaceImplementation();
            property.serializedObject.ApplyModifiedProperties();
        })
        { text = "MyterfaceImplementation" });

        root.Add(new Button(() =>
        {
            p.managedReferenceValue = new MyOtherInterfaceImplementation();
            property.serializedObject.ApplyModifiedProperties();
        })
        { text = "MyOtherInterfaceImplementation" });
        root.Add(new PropertyField(p));
        return root;
    }
}